Hydrocarbon wells are often completed with a production tubing prior to perforating the wellbore for multiple reasons; including, but not limited to, the high pressure seen in the wellbore after the casing has been perforated. It's this high pressure that does not allow the subsequence safe lowering of a production string into a “live” well. Because of this, in a large percentage of hydrocarbon wells the production tubing string is put in place prior to perforating the well. To counteract this high pressure, the well can be controlled or killed with heavy fluids before the tubing is run (or before the perforating gun is run) but this fluid may damage the formation. It is well documented and known in the art that these control or heavy fluids can drastically reduce a well's productivity. Other reasons for perforating after production tubing has been placed in the well may include the cost of a drilling rig. The operator may choose to run production tubing with a drilling rig and replace it with a less costly work over rig for operations like perforating, well clean up, well testing, well hook up, etc. and the work over rig may not have the capacity to safely and efficiently run production tubing into the well so it must be done by the drilling rig.
The service industry has devised ways to perforate a well with a production tubing in place but not without compromising the time and cost needed to perforate the well or the performance of the perforating guns themselves. One can divide the methods currently used to perforate a well with the production tubing in place into two main categories: perforating casing guns that are run before or attached to the production tubing and through tubing perforating guns.
Perforating casing guns can be run and hung in place by a plug like device to be activated at a later time after the production tubing is run. The activation means are varied, it could be pressure activated, time activated, a combination of time and pressure or by lowering a firing head via wireline. Casing guns can also be deployed with the production tubing string by hanging it onto the bottom of the tubing string. In both cases, after the perforating gun is activated the remaining hardware of the casing gun is dropped down to the sump of the well. These two techniques using casing guns allow the operators to use the biggest possible guns, bigger guns usually result in deeper penetration and better flow of hydrocarbons. It will also allow them to leave the proper clean and light fluid in place in the wellbore at the time of perforating to avoid damage to the formation. However some complications and additional work is needed for this type of operation, the well needs to be drilled and cased deeper to account for a sump large enough to house the casing guns that will be dropped after firing. There is also the chance that a casing gun dropping mechanism will not work as intended or that the dropped gun does not drop all the way down to the bottom of the sump and could obstruct the producing interval.
The second alternative is to use through tubing guns. These guns are typically of a smaller diameter than the casing guns as it needs to be able to go through the production tubing string and all of its restrictions of internal diameter. Most of the through tubing guns are of the exposed gun type. Exposed guns were designed without the hollow carrier that typically characterizes casing guns in an effort to use the largest possible shaped charge that will fit a certain restriction of internal diameter. As is widely documented and a person of ordinary skill in the art will know, through tubing guns are optimized to carry the largest possible shape charges as the challenge to create a deep enough perforating tunnel with a small perforating gun are several. As an example, a 5 inch casing will typically be perforated with a 3⅜ inch casing gun but if the production tubing is run before the well is perforated, the through tubing gun normally used will be 2½ inch (for a 5 inch casing size the production tubing string will typically be 3½ inch).
As is widely documented, the “water clearance” or distance from a shaped charge to the inside face of a casing will impact the total penetration of a shaped charge negatively as the water clearance increases. This is one of the reasons, among others, that the use of a smaller perforating gun will typically reduce the depth of penetration of shaped charges facing towards the largest water clearance. To counter this effect, some guns are designed to what is called zero phase, this is all the shaped charges are looking in the same direction, and are oriented with devices like magnets or bowsprings so the shaped charges are facing the casing, thereby minimizing the water clearance. A downside of using zero phased guns, among others, is that the drainage of the reservoir is not as efficient as if a helicoidally or spiral designed gun is used.
Helicoidally or spiral designed guns will transport more charges downhole on a gun of the same length as a zero phased gun and therefore have more area open to flow fluids from the reservoir. As mentioned above, the shaped charges with the largest separation from the casing wall will see its depth of penetration reduced. In an example of a 5 inch casing, with internal diameters typically ranging between 4.560 inches and 4.006 inches, a through tubing perforating gun that typically might be used is a 2½ inch external diameter perforating gun and it is to be assumed that the perforating gun will always be in contact with one side of the casing leaving a large clearance for the shaped charges facing opposite to the casing/perforating gun contact. One type of through tubing gun is described in U.S. Pat. No. 5,816,343 entitled “Phased perforating guns” issued Oct. 6, 1998 to Schlumberger Technology Corporation. Another type of through tubing perforating gun is disclosed in U.S. Pat. No. 6,591,911 entitled “Multi-directional gun carrier method and apparatus” issued Jul. 15, 2003 to Schlumberger Technology Corporation. Another design of through tubing guns aimed to both reduce the water clearance and to lower the biggest possible charge through the restrictions of a production tubing is commonly known in the industry as the “Pivot Gun” and is described in U.S. Pat. No. 5,095,801 entitled “Pivot gun having charges which slidingly engage a stationary detonating cord and apparatus for deploying the charges” by Jorge E. Lopez de Cardenas issued Mar. 17, 1992 and assigned to Schlumberger Technology Corporation.
References to the decrease in depth of penetration a shaped charge can suffer from increased water clearance can be found, among multiple other publications in the public domain and in the knowledge of those skilled in the art, in SPE (Society of Petroleum Engineers) Article No. 27424 entitled “Simple Method Predicts Downhole Shaped Charge Gun Performance” published in August 1994 by R. E. Ott et al.
In an effort to counteract the effects of having to use a small gun in a well with a production tubing in place some operators have come up with innovative ideas, some involve running the perforating gun on the outside of the casing. This method is described in Published U.S. Patent Application 20040206503 entitled “Casing conveyed well perforating apparatus and method” filed May 6, 2004 and assigned to Shell Oil Co.